1. Field of the Invention
The invention relates generally to methods of accelerated curing of condensation polymers or crosslinking of polymers by a direct current glow discharge formed over the polymer bulk. The invention includes methods of forming hydrophilic surfaces, conductive layers on polymer surfaces and rapid, low temperature removal of solvent from a polymerizing matrix. The invention provides an efficient method of crosslinking and drying polymeric compositions.
2. Description of Related Art
Sol-gel techniques have been extensively investigated for more than two decades (Gottardi, 1982) and used to prepare glasses and ceramics for use in a wide variety of applications, employing various precursors, catalysts, additives and procedures.
Following the partial or complete hydrolysis of the silicon-alkoxide precursor, polycondensation of the hydroxyl groups takes place via the Si--O--Si etheric bonds. The glassy matrix formed by this polymerization is capable of encaging large molecules (e.g. chromophores, enzymes) which have been introduced into the reaction mixture. However, during this stage of the sol-gel synthesis severe cracking and fragmentation of the formed glass are the common obstacles that impede the fabrication of articles and films in general, and supported films in particular. This is due to the extensive volume-contraction which accompanies the condensation reactions and the corresponding expulsion of the condensation reactions and the corresponding expulsion of the solvent and the condensation products. Many investigators have attempted to overcome this obstacle by using a wide variety of additives such as dimethylformamide (DMF), formamide, organic acids or surfactants. Even with these additives, however, an extremely slow and very cautious drying of the sol-gel glass is necessary for the survival of a fracture-free glass, making this synthetic route more of an art than a science.
Although crack-free glasses from sol-gels have been prepared, the procedures are time-consuming and often complex. Of particular concern from a practical aspect is the undue amount of time required in curing glass thin-films on supported matrices. It is not unusual to dry polysiloxane films for periods of weeks to assure a quality glass. Such films, depending on conditions of drying, additives and process modifications have required from 6 to 45 days for complete drying to crack-free films (Hara, 1991).
Methods of curing polymers typically have involved the use of heat, UV or ionizing radiation, or chemical modification methods. Surface treatment of plastic films, improving the wettability for example, has been performed employing alternating current (AC) corona discharge techniques. AC corona treatment also appears to lower heat-sealing temperatures for most polymers and is often used to improve adhesion on automotive top coats by transforming a nonpolar topcoat surface into a polar adhesively suitable substrate for structural bonding. Effects of AC corona discharge treatment can generally be erased, either by a heat treatment or by contact with a metallic surface. Upon prolonged storage, slow migration of low molecular weight nonpolar species to the film surface and/or migration of polar species from the surface into the bulk are believed to cause gradual decay of corona-induced wettability properties (Wu, 1982).
Electrolysis using direct current (DC) glow discharge has been reviewed (Hickling, 1971). The method consists of placing an electrode above the surface of a liquid, usually an aqueous electrolytic solution, and the other electrode inside the bulk liquid. At sufficiently high current voltage, an electrical discharge is initiated creating free radicals or ionic species at the solution surface. In general there is little or no specificity in the ensuing electrochemical reactions. Moreover, the solutions must be vigorously stirred in order to dissipate the heat generated in the reactions.